1. Field of the Disclosure
The disclosure relates generally to methods and compounds for Vitamin D therapy. More particularly, the disclosure relates to compositions comprising 1,25-dihydroxyvitamin D2 and methods of administration thereof in the treatment and prevention of disease.
2. Brief Description of Related Technology
Secondary hyperparathyroidism is a disorder which develops primarily because of Vitamin D deficiency. It is characterized by abnormally elevated blood levels of parathyroid hormone (PTH) and, in the absence of early detection and treatment, it becomes associated with parathyroid gland hyperplasia and a constellation of metabolic bone diseases. It is a common complication of chronic kidney disease (CKD), with rising incidence as CKD progresses. Secondary hyperparathyroidism can also develop in individuals with healthy kidneys, due to environmental, cultural or dietary factors which prevent adequate Vitamin D supply.
As to secondary hyperparathyroidism and its occurrence in CKD, there is a progressive loss of cells of the proximal nephrons, the primary site for the synthesis of the vitamin D hormones (collectively “1,25-dihydroxyvitamin D”) from 25-hydroxyvitamin D3 and 25-hydroxyvitamin D2. In addition, the loss of functioning nephrons leads to retention of excess phosphorus which combined reduces the activity of the renal 25-hydroxyvitamin D-1α-hydroxylase, the enzyme which catalyzes the reaction to produce the D hormones. These two events account for the low serum levels of 1,25-dihydroxyvitamin D commonly found in patients with moderate to severe CKD when vitamin D supply is adequate.
Reduced serum levels of 1,25-dihydroxyvitamin D cause increased, and ultimately excessive, secretion of PTH by direct and indirect mechanisms. The resulting hyperparathyroidism leads to markedly increased bone turnover and its sequela of renal osteodystrophy, which may include a variety of other diseases, such as, osteitis fibrosa cystica, osteomalacia, osteoporosis, extraskeletal calcification and related disorders, e.g., bone pain, periarticular inflammation and Mockerberg's sclerosis. Reduced serum levels of 1,25-dihydroxyvitamin D can also cause muscle weakness and growth retardation with skeletal deformities (most often seen in pediatric patients).
“Vitamin D” is a term that refers broadly to the organic substances named Vitamin D2, Vitamin D3, Vitamin D4, etc., and is sometimes used loosely to refer to their metabolites and hormonal forms that influence calcium and phosphorus homeostasis. “Vitamin D deficiency” is a term that broadly refers to reduced or low blood levels of Vitamin D, as defined immediately above.
The most widely recognized forms of Vitamin D are Vitamin D2 (ergocalciferol) and Vitamin D3 (cholecalciferol). Vitamin D2 is produced in plants from ergosterol during sunlight exposure and is present, to a limited extent, in the human diet. Vitamin D3 is generated from 7-dehydrocholesterol in human skin during exposure to sunlight and also is found, to a greater extent than Vitamin D2, in the human diet, principally in dairy products (milk and butter), certain fish and fish oils, and egg yolk. Vitamin D supplements for human use consist of either Vitamin D2 or Vitamin D3.
Both Vitamin D2 and Vitamin D3 are metabolized into prohormones by one or more enzymes located in the liver. The involved enzymes are mitochondrial and microsomal cytochrome P450 (CYP) isoforms, including CYP27A1, CYP2R1. CYP3A4, CYP2J3 and possibly others. These enzymes metabolize Vitamin D2 into two prohormones known as 25-hydroxyvitamin D2 and 24(S)-hydroxyvitamin D2, and Vitamin D3 into a prohormone known as 25-hydroxyvitamin D3. The two 25-hydroxylated prohormones are more prominent in the blood, and can be collectively referred to as “25-hydroxyvitamin D.” Vitamin D2 and Vitamin D3 can be metabolized into their respective prohormones outside of the liver in certain epithelial cells, such as enterocytes, which contain the same (or similar) enzymes, but extrahepatic prohormone production probably contributes little to blood levels of 25-hydroxyvitamin D.
The rates of hepatic and extrahepatic production of the Vitamin D prohormones are not tightly regulated, and they vary mainly with intracellular concentrations of the precursors (Vitamin D2 and Vitamin D3). Higher concentrations of either precursor increase prohormone production, while lower concentrations decrease production. Hepatic production of prohormones is inhibited by high levels of 25-hydroxyvitamin D via a poorly understood mechanism apparently directed to prevention of excessive blood prohormone levels.
The Vitamin D prohormones are further metabolized in the kidneys into potent hormones by an enzyme known as CYP27B1 (or 25-hydroxyvitamin D3-1α-hydroxylase) located in the proximal kidney tubule. The prohormones 25-hydroxyvitamin D2 and 24(S)-hydroxyvitamin D2 are metabolized into hormones known as 1α,25-dihydroxyvitamin D2 and 1α,24(S)-dihydroxyvitamin D2. Likewise, 25-hydroxyvitamin D3 is metabolized into a hormone known as 1α,25-dihydroxyvitamin D3 (or calcitriol). These hormones are released by the kidneys into the blood for systemic delivery. The two 1α,25-dihydroxylated hormones, usually far more prominent in the blood than 1α,24(S)-dihydroxyvitamin D2, can be collectively referred to as “1,25-dihydroxyvitamin D.” Vitamin D prohormones can be metabolized into hormones outside of the kidneys in keratinocytes, lung epithelial cells, enterocytes, cells of the immune system (e.g., macrophages) and certain other cells containing CYP27B1 or similar enzymes, but such extrarenal hormone production is incapable of sustaining normal blood levels of 1,25-dihydroxyvitamin D in advanced CKD.
Blood levels of 1,25-dihydroxyvitamin D are precisely regulated by a feedback mechanism which involves PTH. The renal 1α-hydroxylase (or CYP27B1) is stimulated by PTH and inhibited by 1,25-dihydroxyvitamin D. When blood levels of 1,25-dihydroxyvitamin D fall, the parathyroid glands sense this change via intracellular Vitamin D receptors (VDR) and secrete PTH. The secreted PTH stimulates expression of renal CYP27B1 and, thereby, increases production of Vitamin D hormones. As blood concentrations of 1,25-dihydroxyvitamin D rise again, the parathyroid glands attenuate further PTH secretion. As blood PTH levels fall, renal production of Vitamin D hormones decreases. Rising blood levels of 1,25-dihydroxyvitamin D also directly inhibit further Vitamin D hormone production by CYP27B1.
PTH secretion can be abnormally suppressed in situations where blood 1,25-dihydroxyvitamin D concentrations become excessively elevated, as can occur in certain disorders such as sarcoidosis or as a result of bolus doses of Vitamin D hormone replacement therapies. Oversuppression of PTH secretion can cause or exacerbate disturbances in calcium homeostasis. The parathyroid glands and the renal CYP27B1 are exquisitely sensitive to changes in blood concentrations of Vitamin D hormones so that serum 1,25-dihydroxyvitamin D is tightly controlled, fluctuating up or down by less than 20% during any 24-hour period. In contrast to renal production of Vitamin D hormones, extrarenal production is not under precise feedback control.
Blood levels of 1,25-dihydroxyvitamin D and substrate 25-hydroxyvitamin D prohormone, and regulation thereof, can also be affected by vitamin D hormone analogs, such as 1α-hydroxyvitamin D2 and 19-nor-1,25 dihydroxyvitamin D2.
The Vitamin D hormones have essential roles in human health which are mediated by the intracellular VDR. In particular, the Vitamin D hormones regulate blood calcium levels by controlling intestinal absorption of dietary calcium and reabsorption of calcium by the kidneys. The Vitamin D hormones also participate in the regulation of cellular differentiation and growth and normal bone formation and metabolism. Further, Vitamin D hormones are required for the normal functioning of the musculoskeletal, immune and renin-angiotensin systems. Numerous other roles for Vitamin D hormones are being postulated and elucidated, based on the documented presence of intracellular VDR in nearly every human tissue. For example, vitamin D has been postulated to play a role in cellular differentiation and cancer, in regulation of the immune system (immune enhancing or immune suppressing effects, depending on the situation), and atherosclerosis. Vitamin D deficiency increases the risk of many common cancers, multiple sclerosis, rheumatoid arthritis, hypertension, cardiovascular heart disease, and type I diabetes.
The actions of Vitamin D hormones on specific tissues depend on the degree to which they bind to (or occupy) the intracellular VDR in those tissues. VDR binding increases as the intracellular concentrations of the hormones rise, and decreases as the intracellular concentrations fall. In all cells, intracellular concentrations of the Vitamin D hormones change in direct proportion to changes in blood hormone concentrations. In cells containing CYP27B1 (or similar enzymes), intracellular concentrations of the Vitamin D hormones also change in direct proportion to changes in blood and/or intracellular prohormone concentrations, as discussed above.
Vitamin D2, Vitamin D3 and their prohormonal forms have affinities for the VDR which are estimated to be at least 100-fold lower than those of the Vitamin D hormones and do not effectively activate the receptor. As a consequence, physiological concentrations of these hormone precursors exert little, if any, biological actions without prior metabolism to Vitamin D hormones. However, supraphysiological levels of these hormone precursors, especially the prohormones, in the range of 10 to 1,000 fold higher than normal, can sufficiently occupy the VDR and exert actions like the Vitamin D hormones.
Blood levels of Vitamin D2 and Vitamin D3 are normally present at stable concentrations in human blood, given a sustained, adequate supply of Vitamin D from sunlight exposure and an unsupplemented diet. Slight, if any, increases in blood Vitamin D levels occur after meals since unsupplemented diets have low Vitamin D content, even those containing foods fortified with Vitamin D. The Vitamin D content of the human diet is so low that the National Institutes of Health (NIH) cautions “it can be difficult to obtain enough Vitamin D from natural food sources” [NIH, Office of Dietary Supplements, Dietary Supplement Fact Sheet: Vitamin D (2005)]. Almost all human Vitamin D supply comes from fortified foods, exposure to sunlight or from dietary supplements, with the last source becoming increasingly important. Blood Vitamin D levels rise only gradually, if at all, after sunlight exposure since cutaneous 7-dehydrocholesterol is modified by UV radiation to pre-Vitamin D3 which undergoes thermal conversion in the skin to Vitamin D3 over a period of several days before circulating in the blood.
Blood Vitamin D hormone concentrations also remain generally constant through the day in healthy individuals, but can vary significantly over longer periods of time in response to seasonal changes in sunlight exposure or sustained alterations in Vitamin D intake. Marked differences in normal Vitamin D hormone levels are commonly observed between healthy individuals, with some individuals having stable concentrations as low as approximately 20 pg/mL and others as high as approximately 70 pg/mL. Due to this wide normal range, medical professionals have difficulty interpreting isolated laboratory determinations of serum total 1,25-dihydroxyvitamin D; a value of 25 pg/mL may represent a normal value for one individual or a relative deficiency in another.
Transiently low blood levels of 1,25-dihydroxyvitamin D stimulate the parathyroid glands to secrete PTH for brief periods ending when normal blood Vitamin D hormone levels are restored. In contrast, chronically low blood levels of 1,25-dihydroxyvitamin D continuously stimulate the parathyroid glands to secrete PTH, resulting in a disorder known as secondary hyperparathyroidism. Chronically low hormone levels also decrease intestinal calcium absorption, leading to reduced blood calcium concentrations (hypocalcemia) which further stimulate PTH secretion. Continuously stimulated parathyroid glands become increasingly hyperplastic and eventually develop resistance to regulation by vitamin D hormones. Without early detection and treatment, secondary hyperparathyroidism progressively increases in severity, causing debilitating metabolic bone diseases, including osteoporosis and renal osteodystrophy.
Chronically low blood levels of 1,25-dihydroxyvitamin D develop when there is insufficient renal CYP27B1 to produce the required supply of Vitamin D hormones, a situation which commonly arises in CKD. The activity of renal CYP27B1 declines as the Glomerular Filtration Rate (GFR) falls below approximately 60 ml/min/1.73 m2 due to the loss of functioning nephrons. In end-stage renal disease (ESRD), when the kidneys fail completely and hemodialysis is required for survival, renal CYP27B1 often becomes altogether absent. Any remaining CYP27B1 is greatly inhibited by elevated serum phosphorous (hyperphosphatemia) caused by inadequate renal excretion of dietary phosphorous.
Chronically low blood levels of 1,25-dihydroxyvitamin D also develop because of a deficiency of Vitamin D prohormones, since renal hormone production cannot proceed without the required precursors. Prohormone production declines markedly when cholecalciferol and ergocalciferol are in short supply, a condition often described by terms such as “Vitamin D insufficiency,” “Vitamin D deficiency,” or “hypovitaminosis D.” Therefore, measurement of 25-hydroxyvitamin D levels in blood has become the accepted method among healthcare professionals to monitor Vitamin D status. Recent studies have documented that the great majority of CKD patients have low blood levels of 25-hydroxyvitamin D, and that the prevalence of Vitamin D insufficiency and deficiency increases as CKD progresses.
It follows that individuals most vulnerable to developing chronically low blood levels of 1,25-dihydroxyvitamin D are those with CKD. Most CKD patients typically have decreased levels of renal CYP27B1 and a shortage of 25-hydroxyvitamin D prohormones. Not surprisingly, most CKD patients develop secondary hyperparathyroidism. Unfortunately, early detection and treatment of secondary hyperparathyroidism in CKD is rare, let alone prevention.
The National Kidney Foundation (NKF) has recently focused the medical community's attention on the need for early detection and treatment of secondary hyperparathyroidism by publishing Kidney Disease Outcomes Quality Initiative (K/DOQI) Clinical Practice Guidelines for Bone Metabolism and Disease in Chronic Kidney Disease [Am. J. Kidney Dis. 42:S1-S202, 2003)]. The K/DOQI Guidelines identified the primary etiology of secondary hyperparathyroidism as chronically low blood levels of 1,25-dihydroxyvitamin and recommended regular screening in CKD Stages 3 through 5 for elevated blood PTH levels relative to Stage-specific PTH target ranges. CKD Stage 3 was defined as moderately decreased kidney function (GFR of 30-59 mL/min/1.73 m2) with an intact PTH (iPTH) target range of 35-70 pg/mL; Stage 4 was defined as severely decreased kidney function (GFR of 15-29 mL/min/1.73 m2), with an iPTH target range of 70-110 pg/mL; and Stage 5 was defined as kidney failure (GFR of <15 mL/min/1.73 m2 or dialysis) with an iPTH target range of 150-300 pg/mL. In the event that screening revealed an iPTH value to be above the ranges targeted for CKD Stages 3 and 4, the Guidelines recommended a follow-up evaluation of serum total 25-hydroxyvitamin D to detect possible Vitamin D insufficiency or deficiency. If 25-hydroxyvitamin D below 30 ng/mL was observed, the recommended intervention was Vitamin D repletion therapy using orally administered ergocalciferol. If 25-hydroxyvitamin D above 30 ng/mL was observed, the recommended intervention was Vitamin D hormone replacement therapy using known oral or intravenous Vitamin D hormones or analogs. The Guidelines did not recommend the concurrent application of Vitamin D repletion and Vitamin D hormone replacement therapies, consistent with warnings mandated by the Food and Drug Administration in package inserts for Vitamin D hormone replacement products.
The NKF K/DOQI Guidelines defined Vitamin D sufficiency as serum 25-hydroxyvitamin D levels ≧30 ng/mL. Recommended Vitamin D repletion therapy for patients with “Vitamin D insufficiency,” defined as serum 25-hydroxyvitamin D of 16-30 ng/mL, was 50,000 IU per month of oral Vitamin D2 for 6 months, given either in single monthly doses or in divided doses of approximately 1,600 IU per day. Recommended repletion therapy for patients with “Vitamin D deficiency” was more aggressive: for “mild” deficiency, defined as serum 25-hydroxyvitamin D of 5-15 ng/mL, the Guidelines recommended 50,000 IU per week of oral Vitamin D2 for 4 weeks, followed by 50,000 IU per month for another 5 months; for “severe” deficiency, defined as serum 25-hydroxyvitamin D below 5 ng/mL, the Guidelines recommended 50,000 IU/week of oral Vitamin D2 for 12 weeks, followed by 50,000 IU/month for another 3 months. Doses of 50,000 IU per week are approximately equivalent to 7,000 IU per day.
Most concepts of vitamin D metabolism and function have been developed with the rat and/or chick as experimental models. Studying vitamin D metabolism is hampered by the paucity of data on the normal circulating levels of vitamin D metabolites in mammals under normal conditions. Most recent research has focused on the analysis of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D as indicators of vitamin D status or aberrant physiological states.
Shortly after the discovery of vitamin D2 it seemed apparent that Vitamins D2 and D3 had similar biological activities in most mammals. More recent research, fostered by the discovery of sensitive analytical techniques and the availability of high specific activity 3H-labeled vitamin D species, indicated that differences in the metabolism of Vitamins D2 and D3 in mammals are perhaps widespread. Most notable were the apparent discrimination against Vitamin D2 by pigs [Biochem J. 204:185-189], cows [J Nutr 113:2595-2600], and humans [Gene Regulation, Structure-Function Analysis and Clinical Application, Walter de Gruyter. Berlin, pp. 765-766] and the apparent preference of Vitamin D2 by rats [Biochem J 204:185-189, J Bone Miner Res 5(Supplement 2):S265].
Vitamin D and its metabolites are transported in the blood of vertebrates attached to Vitamin D binding protein (DBP). Baird et al [Recent Prog Horm Res. 25:611-664] have shown that protein binding increases the solubility of steroids and that the metabolic clearance rate of steroids is in part dependent on their binding to specific plasma proteins.
Hay and Watson [Comp Biochem Physiol 56B:375-380] studied the affinities of DBP for 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 in 63 vertebrate species. They found that many of the studied species discriminated against 25-hydroxyvitamin D2 in favor of 25-hydroxyvitamin D3 [Biochem J 204:185-189]. However, in rats the discrimination is against Vitamin D3 in favor of Vitamin D2. The rat DBP is known to have equal affinity for 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3, but a lower affinity for Vitamin D2 relative to Vitamin D3 [Steroids 37:581-592]. Reddy et al., [Calci Tissue Int 36:524] suggested that the lower affinity for Vitamin D2 resulted in its enhanced availability for liver 25-hydroxylation. Hence, in the presence of DBP, more 25-hydroxyvitamin D2 was made relative to 25-hydroxyvitamin D3 when equal amounts of Vitamin D2 or Vitamin D3 substrate were perfused into rat livers. In the experiments conducted by Reddy et al., if binding protein was eliminated from the perfusion media, equal amounts of 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 were synthesized. Collectively, these data suggest that discrimination against the different forms of Vitamin D could likely result from variations in the affinity of DBP for the parent compound and/or one or more of their metabolites. Regardless of the mechanism for discrimination, it appears that these differences are present to afford the species the most efficient utilization of the most abundant Vitamin D metabolites available in their environment.
Critical questions remain unanswered regarding complete elucidation of the Vitamin D2 metabolic pathway, and species differences between Vitamin D2 and D3 metabolism are still virtually unexplored. The introduction of Vitamin D as a pharmacological intervention has resulted in a totally different set of issues regarding their metabolism, tissue kinetics, mechanism of action, and potential therapeutic uses.
Vitamin D receptors are present throughout the human body in a wide variety of cells, and there have been reports that vitamin D hormone has diverse “non-classical” biologic effects on cellular proliferation, the immune system and the cardiovascular system, beyond its “classical” effects on the PTH system. It has also been reported that 25-hydroxyvitamin D2 has direct effects on parathyroid cells in suppressing PTH [Kidney International, 70(4):654-659, August 2006]. There has been one report that Vitamin D2 was less than one-third as potent as Vitamin D3 and exhibited a shorter duration of action relative to Vitamin D3; administration of 50,000 IU of ergocalciferol or cholecalciferol to healthy male humans produced similar rises in serum concentration of the administered vitamin, indicating equivalent absorption, but 25-hydroxyvitamin D3 levels peaked at 14 days whereas 25-hydroxyvitamin D2 levels fell early and were not different from baseline at 14 days [J. Clin. Endocrinol. Metab., 89(11):5387-5391 (2004)].
Thus, the relative contribution of 25-hydroxyvitamin D compounds and 1,25-dihydroxyvitamin D compounds to PTH suppression, the relative potency of 1,25-dihydroxyvitamin D2 and 1,25-dihydroxyvitamin D3 in vivo, and the spectrum of non-classical biological effects of each of these hormones has not clearly been elucidated. There remains a need for alternative vitamin D hormone therapies that ideally provide beneficial effects on PTH levels, immune status and/or cardiovascular health, with reduced toxicity.